Ultraviolet extinction in the solar atmosphere

An effort was made to reproduce the observed solar continuum central intensity and Eddington flux by treating important bound-free absorbers, diatomic molecular bands, and atomic lines. Emphasis was placed on the 0.5 to 0.15 μm ultraviolet region. Predictions were made with the stellar atmosphere computer program ATLAS5 modified to incorporate the new absorption sources. Two empirical solar atmospheric structures were employed: the HSRA model of Gingerich, Noyes, Kalkofen and Cuny and a recent model (VAL) of Vernazza, Avrett and Loeser. Bound-free absorption was augmented through quantum-defect and hydrogenic excited-term cross sections for neutral C, Mg, Al and Si, a measured ground state Al I cross section, and twenty-six theoretical neutral Fe term cross sections calculated by A. Merts. Thirteen band systems of electronic transitions in diatomic molecules contributed significantly. The molecular absorption was computed by Golden's smeared-line approximation. Localized regions of strong ultraviolet absorption were found to be due to CH, NH, OH, CN and SiO. The CO 4th positive system dominated the region 0.17 to 0.2 μm. Several visible and infrared bands of MgH, C2, CN and CO were included. The 0.5 to 0.2 μm region was influenced strongly by absorption lines of iron-group elements. Observational agreement of final intensity and flux predictions for both the HSRA and VAL model structures appears to be substantially complete; only small spectral intervals require additional attention